V.

Work in Astronomy

During most of his time in Padua, Galileo showed little interest in astronomy, although in 1595 he declared in a letter that he preferred the Copernican theory that Earth revolves around the Sun to the assumptions of Aristotle and Ptolemy that planets circle a fixed Earth (see Astronomy: The Copernican Theory; Ptolemaic System).

A.

Observations with the Telescope

In 1609 Galileo heard that a telescope had been invented in Holland. In August of that year he constructed a telescope that magnified about ten times and presented it to the doge of Venice. Its value for naval and maritime operations resulted in the doubling of his salary and his assurance of lifelong tenure as a professor.

By December 1609 Galileo had built a telescope of 20 times magnification, with which he discovered mountains and craters on the Moon. He also saw that the Milky Way was composed of stars, and he discovered four satellites circling Jupiter. It was therefore undeniable that at least some heavenly bodies move around a center other than Earth, a finding that did not prove that Copernicus had been right but did fit in well with the Copernican system of the universe. Galileo published these findings in March 1610 in a book called The Starry Messenger.

Galileo astutely used his new fame to secure an appointment for which he had been angling for some time, that of court mathematician at Florence, Italy. He was thereby freed from teaching duties and had time for research and writing. By December 1610 he had observed the phases of Venus and found that variations in the planet’s brightness were much greater than previously thought. These could be explained as a natural consequence of the Copernican system but not by the Ptolemaic system. Galileo naturally took the discovery of Jupiter’s moons and his observations of Venus as confirmation of the Copernican system.

Traditionalist professors of philosophy scorned Galileo’s discoveries because Aristotle had held that only perfectly spherical bodies could exist in the heavens and that their movement was eternal and circular. This view could not be maintained if Venus, for example, was sometimes nearer Earth and sometimes farther away. Nor could Aristotelian theory explain why Venus sometimes appears crescent-shaped, like the Moon. Galileo also disputed with professors at Florence and Pisa over hydrostatics, and he published a book on floating bodies in 1612. Four printed attacks on this book followed, rejecting Galileo’s physics. Aristotelians took shape to be the key to explaining why bodies float, whereas Galileo relied on the relative densities of the floating object and the medium in which it floated. In 1613 Galileo published a work on sunspots and predicted victory for the Copernican theory.

B.

Theory of Tides

Only the Copernican model supported Galileo’s ingenious but mistaken theory of tides. According to Galileo’s theory, the motion of Earth’s rotation is alternately added to Earth’s orbital motion and subtracted from it, with the effect that the seas are set sloshing backward and forward. To this simple mechanism, which provided one tide every 24 hours, Galileo had to add further factors, such as the orientation and configuration of seabeds and shores, to make a reasonable approximation of the variety of tidal phenomena actually observed at different places and seasons.